1. Field of the Invention
The present invention relates to a process and apparatus for fabricating a semiconductor device, in particular, a semiconductor device comprising a compound semiconductor layer formed by molecular beam epitaxy (MBE).
2. Description of the Prior Art
In the fabrication of semiconductor devices, in particular, compound semiconductor devices, semiconductor layers with different widths of forbidden bands, conduction types, or carrier concentrations are epitaxially grown on a semiconductor substrate to form a multilayer structure. This epitaxial growth may be effected by MBE, liquid phase epitaxy, vapor phase epitaxy, including organic-metal-pyrolysis vapor phase epitaxy, etc.
MBE allows precise control of the composition, the amount of doped impurity, and the growth rate of the semiconductor crystal layer; sharp change of the profile of the composition of a crystal layer or the amount of a doped impurity, for example, a width of change of approximately 1 nm; and growth of a crystal layer with a composition different from one obtained in the state of chemical equilibrium, not possible with liquid phase epitaxy. MBE is therefore a preferred technique for forming a semiconductor crystal multilayer structure with extremely thin layers and very sharp changes of composition at the interfaces of the layers, as in a superlattice device.
The surface of a substrate on which a crystal layer is to be formed by MBE should be kept clean to enable full use of the above-mentioned features of MBE. When the substrate is a crystal layer of gallium arsenide (GaAs), the substrate can be heated in advance to remove the oxide layer formed on the surface of the GaAs layer due to exposure to the atmosphere. However, when the substrate is aluminum gallium arsenide (AlGaAs), for example, the surface oxide layer formed on the surface of the substrate is aluminum oxide (Al.sub.2 O.sub.3), a stable oxide difficult to remove by heat treatment. Such a thermally stable surface oxide layer may be removed by milling by, for example, argon (Ar) ions. This, however, constitutes physical etching, which damages the surface of the crystal substrate, and therefore cannot be used when fabricating, for example, a heterojunction-type field effect transistor (FET). In place of physical etching, use of dry chemical etching may be considered to remove the thermally stable surface oxide layer. Such etching, however, requires use of an etchant gas including chlorine (Cl), fluorine (F), carbon (C), or the like. Such a gas may damage an extremely high vacuum pump, such as an ion pump or a cryo pump, resulting in decreased power of evacuation after, for example, three months daily use, an extremely high vacuum being essential for MBE. It may also contaminate an MBE apparatus, resulting in corrosion. Therefore, in practice, dry chemical etching cannot be used in combination with MBE.
If MBE is effected onto an AlGaAs single crystal substrate without cleaning the surface thereof, an amorphous layer will be formed on the substrate. In the case of a GaAs single crystal layer and an AlGaAs single crystal layer superposed thereon with an opening exposing the surface of the GaAs layer, MBE will not form a single crystal layer fully in the opening since an amorphous layer is formed from the surface of the AlGaAs wall since the surface oxide layer of the AlGaAs layer could not be removed.
Problems also exist in the condition of the surface of a layer formed by MBE. In the conventional process for fabricating a semiconductor device, after the MBE is effected, the substrate and the crystal layer formed thereon by the MBE are cooled and then discharged from the MBE apparatus into the atmosphere before forming the semiconductor elements in and on the MBE layer.
In this prior art process, heat treatment after MBE deteriorates the properties of an active region in MBE layers near the top surface. Such deterioration results from diffusion of an impurity into the active region and should be prevented to maintain the excellent properties of the MBE layers.
For example, MBE is often used for the fabrication of heterojunction FET's. Here, MBE is used to form a nondoped GaAs layer and then a silicon-doped AlGaAs layer on a semi-insulating GaAs substrate. The interface of the GaAs layer and the GaAlAs layer is a heterojunction. An electron accumulating layer (two dimensional electron gas) is formed by displacement of electrons from the n-type AlGaAs layer (an electron supply layer) to the nondoped GaAs layer. If the electron sheet concentration of the electron storage layer is controlled by a voltage applied to a gate electrode formed on the n-type AlGaAs layer, the impedance of the conducting channel in the electron storage layer between a source electrode and a drain electrode is controlled. As a result, the device functions as a transistor.
To form ohmic contact regions of the source and drain electrodes of such a heterojunction-type FET, the following steps are often taken: selectively implanting silicon ions into multiple n-type AlGaAs and nondoped GaAs layers; forming a surface protective layer on the n-type AlGaAs layer; and heating at 700.degree. C. to 800.degree. C. to activate the ion-implanted regions. This type of heat treatment lowers the electron mobility, however, for example, from approximately 110,000 cm.sup.2 /V.sec immediately after MBE to approximately 70,000 cm.sup.2 /V.sec after heat treatment at 700.degree. C. for 15 minutes. The lowering of the electron mobility results from diffusion of silicon (Si) ions from the n-type AlGaAs layer into the nondoped GaAs layer. This lowering of the electron mobility by heat diffusion of an impurity is an important problem in that it negates the effect of providing a heterojunction interface for three dimensionally separating an impurity-doped layer for generating carriers and a channel layer.